As mentioned in U.S. Pat. No. 5,546,791 of Meeten, the rheology or thixotropy of granular fluid slurries, such as cement slurries or drilling mud, can be useful for determining the behaviour of the resultant material. The current standard rheology measurement, as specified by the American Petroleum Industry (API), involves testing of samples in a cup container, and placing a rotor/stator device into the cup. The rotor/stator device comprises a cylindrical, motor-driven rotor having a vaned stator coaxially mounted on the rotation axis. Rotation of the rotor applies torque to the stator. Rate of rotation is compared to the applied torque to determine rheological characteristics of the cup-contained sample.
Meeten observed that rheometers may be inaccurate due to so-called end effects, such as “slippage at the wall” of particle-containing liquid in the container cup, and such as “particle migration” towards the bottom of the cup due to gravity. End effects arise from the torque generated by flow from the ends of the concentric cylinders that comprise the rotor/stator arrangement. Such effects can lead to significant inaccuracies in calculating shear forces applied to the sample fluid, and, hence, its rheological properties.
The first phenomenon known as “slippage at the wall” occurs when the fluid suspension is depleted of particles near the walls of the mixing container. This can, in turn, reduce significantly the measured viscosity and yield stress, consequently leading to large errors in measured values.
The second phenomenon, known as “particle migration,” is primarily caused by gravity, as mentioned above, and also by centrifugation effects. The effect of gravity is to reduce the particle content in the sample zone where the yield stress is measured, and therefore the inaccuracy for rheological measurement is increased for samples mixed using high shear forces. Furthermore, the build-up of solids below the rotor/stator device may cause extra torque to be transmitted to the stator, further increasing error. Meeten further observed that centrifugation was particularly apparent in the external rotor arrangement specified by API. This centrifugation effect decreased the number of particles near the inside of the rotor, thus increasing the inaccuracy of the torque measurement and data obtained.
Meeten further observed that various attempts had been made to reduce one or more of the effects outlined above. These attempts include the provision of a pump and baffle arrangement, for example, as described in SPE Paper Production Engineering, November 1990, pp 415–424. In this publication, the authors Shah and Sutton provided slots at the upper end of the rotor and a helical flange around the outside of the rotor. These were intended to provide flow through the rheometer and to prevent build-up of solids on layers in the region of the rotor/stator.
In the afore-mentioned U.S. Pat. No. 5,546,791, Meeten sought a different way to mitigate the detrimental “end effects” discussed above. However, his remedy remained focused on the use of co-axial rotating cylinders, as employed in the outer rotor/inner stator arrangement (known as the “Couette geometry”) and in the inner rotor/outer stator arrangement (known as the “Searle geometry”). Meeten's rheometer, while retaining a cup container and coaxial rotor/stator arrangement, introduced the use of a pump for providing a pulsatile, non-laminar flow intended for maintaining the particles in suspension while inducing a rheology effect for dispersing and maintaining relative dispersion of the particles and for remaining independent of the measuring system itself.
The present inventors believe that the use of a recirculating pump, as taught by Meeten, could introduce problems for evaluating relatively stiff and viscous mixes such as concrete and mortar. This is because such cementitious mixes, which begin to harden upon hydration, can potentially foul his hoses and pump equipment.
Moreover, it is possible that the effect of high viscous fluids in the hoses, such as cement slurries, pastes, or mortars, might tend to hinder the use of pulsing for dispersing the particles.
Accordingly, the present inventors believe that a novel device is needed for mixing and rheological measurement, one that is particularly capable of assessing the rheology of high viscosity fluids, and, especially, for particle suspensions such as cement slurries, pastes, and mortars.